Vibration sensors are used for monitoring structures (e.g., buildings, bridges), vehicles (e.g., ships, airplanes, autos, trains) and machines. In fabrication facilities, many tools, particularly high precision lithographic and analytical tools, require real time monitoring for any abnormal vibration signals. Excess vibrations in the environment may cause malfunction of tools. Vibration signals originating from the tools themselves may serve as an indication that the tools require maintenance. Accurate in-line monitoring could significantly reduce down time and its associated cost. Machine vibration monitoring and diagnostics require a very low noise floor at low frequencies because the amplitude of vibrations, in terms of acceleration that is produced, is very small at low frequencies.
However, state of the art vibration sensors available on the market cannot satisfy all of these applications. Highly sensitive vibration sensors with a noise floor of approximately 1 μg/sqrt(Hz) at approximately 10 Hz are large in size, making them difficult to use in many applications requiring a small form factor and light weight, such as monitoring robotic arms. They are also prohibitively expensive for large-scale sensor network applications that are becoming increasingly important.
Microelectromechanical systems (MEMS) accelerometers based on piezoelectric sensing have been demonstrated to be an attractive alternative to capacitive sensing because of lower power consumption (use of active materials) and potential integration of sensing and actuating elements within one device.
Currently MEMS based sensors of small sizes are not sensitive enough at low frequency ranges. A MEMS sensor covering a reasonable frequency range typically has its resonance frequency greater than 10 KHz. Such a sensor has a noise level of approximately 100 μg/sqrt(Hz) at approximately 10 Hz which is two orders of magnitude too high.
One architecture to achieve the aforementioned requirements of low noise, wide bandwidth, and small size is based on the approach of a Vibration Spectrum Sensor Array (VSSA).
One bulk-micromachined accelerometer uses CMOS compatible piezoelectric Aluminum Nitride (AlN) thin films and silicon-on-insulator (SOI) wafers. However, one such sensor only showed a sensitivity of 0.45 pC/g and noise floor of 3 μg/sqrt(Hz) at 20 Hz. In order to further improve the signal-to-noise ratio in the low frequency region, a new accelerometer design is needed to obtain an optimized performance in terms of sensitivity per area.